"But" is one of the coordinating conjuctions (For, And, Nor, But, Or, Yet, So) and indicates a contrast between two independent clauses.
Example:
"Waited so patiently for our report BUT we have not submitted it yet"
IMPORTANT:
An independent clause is one in which a complement is not necessary to make sense. If you take the sentences before and after the conjunctions, they make sense by themselves; there is no need of complement:
- Waited so patiently for our report
- We have not submitted it yet.